Water may be supplied to multi-unit structures such as hotels and apartment buildings, using a recirculating system supplied with water from a commercial tank-type water heater. Such a water heater typically includes an Aquastat that has a sensor that senses water temperature within the tank and a control that can be set to a particular minimum water temperature. The control may be set to 140.degree. F. to assure all units receive water at a sufficient temperature such as 110.degree. F. even during heaviest demand. During times of very low demand, a tank temperature such as 115.degree. F. would be sufficient to supply adequate hot water, while avoiding the large heat losses to the environment that occur during recirculating of very hot water.
An earlier U.S. Pat. No. 4,522,333, owned by assignee of the present application, describes an improved system where the temperature T.sub.1 at the water tank outlet is adjusted according to anticipated demand for water. In that system anticipated demand is derived from the history of water usage for that structure, based on demand for water one week earlier, on the same day and at the same time. While such a system can save considerable amounts of heat, a practical system for keeping track of usage during many week-ago periods requires fairly complicated digital circuitry. A hot water heating system which reduced heat loss caused by high water tank temperatures during extended periods of low demand, while providing adequate hot water during periods of high demand, using relatively simple, low cost, and reliable circuitry, would be of considerable value.